In the semiconductor industry, optical inspection systems which, e.g., measure the position of mask structures or so-called “registration marks” on a lithography mask are used to measure lithography masks. This can be implemented with an accuracy in the nanometer range, but only specifies the position of the structures relative to one another.
A further object of inspection systems consists of establishing the position of these structures or marks relative to the edges of the lithography mask. This is referred to as edge detection or “mask edge detection.” Here, for example, an accuracy of the order of 10 μm is sought after.
A problem occurring when detecting the edges consists of the fact that, when using the optical system provided for measuring the mask structures, this edge cannot be measured directly since lithography masks usually have chamfers, i.e., angled edges. Consequently, the actual edge of the lithography mask no longer lies in the depth of field of the optical unit and refocusing is often not possible for installation space reasons. For conventional dimensions of the chamfers and when a conventional imaging optical unit for inspecting mask structures is used, an edge unsharpness of the order of several hundred μm is to be expected.
In order to get a handle on this problem, e.g., DE 198 17 714 C5 discloses a method for measuring structures on a mask surface, which method comprises the measurement of a coordinate position of two outer edges of the mask perpendicular to one another. To this end, use is made of an imaging optical unit with a small aperture. Here, the measuring table surface has a reflecting embodiment for the imaging rays of the measuring instrument, at least in the region of the outer edges of the mask placed thereupon, as a result of which the edge is illuminated in the reflected light.
However, the disadvantage of this method is that the surface of the measuring table must be processed in order to make it reflective. By way of example, if masks with different dimensions are intended to be examined, it is necessary to ensure that the employed measuring table is reflective at the edge of the mask to be measured in each case. If, furthermore, it is not intended for the whole table to have a reflecting configuration, for example in order to minimize stray light which could adversely affect the structure measurements, the reflecting regions of the measuring table should be adapted individually for each mask size. In any case, this procedure is laborious and time-consuming.